1. Field of the Invention
The present invention relates to a motor-driven power steering system for vehicles such as automobiles, and more particularly to a motor-driven power steering system having a steering servo unit including an electric motor for producing assistive steering torque.
2. Description of the Relevant Art
Various electric or motor-driven power steering systems for automobiles have been proposed in recent years in view of the structural complexities of conventional hydraulically operated power steering systems.
One example of such an automotive motor-driven power steering system is disclosed in UK patent application No. 2,135,642 A published on Sept. 5, 1984. The disclosed motor-driven power steering system has a steering servo unit using a low-torque, high-speed electric motor as a power source and a control apparatus for the steering servo unit. When a steering wheel is turned, the steering torque applied to the input shaft of the steering system which is coupled to the steering wheel is detected, and the motor is controlled by the detected steering torque. In low- and medium-speed ranges, assistive torque is produced by the motor and transmitted via a speed reducer to the output shaft of the steering system. The speed reduction ratio of the speed reducer is selected to be high since the motor rotates at high speed. The assistive torque applied to the output shaft of the steering system helps the driver turn the steering wheel with reduced manual forces, resulting in improved drivability and steering feeling. At a high vehicle speed, the armature winding of the motor is short-circuited at a ratio proportional to the steering torque Ts. Therefore, a damping force is produced in proportion to the steering torque Ts, so that larger steering reactive forces are generated than possible with a manually operated steering system.
In most cases, while a steerable or dirigible wheel, which may be a front wheel, is being steered in one direction for the automobile to make a turn, the front wheel is subjected to a returning force Fr that tends to move the front wheel back to its neutral position. The returning force Fr arises from front wheel alignment and also a self-aligning torque produced by elastic deformation of the front wheel. The returning force Fr is transmitted to the steering wheel as a road-induced load. The returning force Fr is low when the vehicle speed Vs is low, and increases as the vehicle speed Vs goes higher.
The above power steering system basically has two steering conditions or modes: (i) The steering torque Ts applied to the steering wheel is larger than a prescribed level. (ii) The applied steering torque Ts is substantially zero. The condition (i) includes (i-1) a positive steering state in which the direction in which the steering torque Ts is applied and the direction in which the dirigible wheel rotates are the same and (i-2) a steering wheel returning state in which the direction of the steering torque Ts and the direction of rotation of the dirigible wheel are not the same. In the state (i-1), the sum of the steering torque Ts and the output torque of the motor is larger than the road-induced load Fr. In the state (i-2), the sum of the steering torque Ts and the output motor torque is smaller than the road-induced load Fr. The condition (ii) may occur when the vehicle is running straight, for example. The steering torque Ts is also zero when substantially no steering force is applied by the driver to the steering wheel, with the driver's hands on or off the steering wheel, for some reason after the dirigible wheel has been steered a certain angle from its neutral position. When this happens, the dirigible wheel starts to return to its neutral position under the returning force F. At the same time, the steering wheel starts returning to its neutral position. This condition will hereinafter be referred to as a freely returning state of the steering wheel. This freely returning state is included in the condition (ii).
In the power steering system disclosed in the above British Patent Application, the steering angle .theta. in the freely returning state of the steering wheel varies as follows: It is assumed that the driver stops applying the steering force to the steering wheel when it has been turned an angle .theta.i clockwise, for example, from its neutral position (.theta.=0) while the vehicle is running at a certain speed. At this time, the freely returning state of hte steering wheel is initiated. The steering wheel repeats overshooting from the neutral position until finally it settles into the neutral position. In the freely returning state of the steering wheel, the motor acts as a load on the dirigible wheel since the motor is rotated through the speed reducer from the side of the dirigible wheel. As a result, the rate of change of the steering angle per unit time is smaller than that of a manually operated steering system. Stated otherwise, the period of reciprocating angular movement of the steering wheel is longer than that in the manually operated steering wheel. Moreover, because the moment of inertia of the motor acts on the dirigible wheel at a rate proportional to the square of the speed reduction ratio of the speed reducer, the extent of overshooting of the steering wheel from the neutral position thereof is larger than that of the manually operated steering system. As a consequence, in the freely returning state of the steering wheel, the stability of returning of the steering wheel to the neutral position is lowered. This problem manifests itself in medium- and high-speed ranges inasmuch as the returning force Fr for the front wheel is larger as the vehicle speed Vs is higher. In a low-speed range, the returning force Fr is smaller in a low-speed range, and hence the returning movement of the steering wheel to its neutral position is assisted by the inertial moment of the motor, thus improving the steering wheel returning stability. As described above, if the steering torque Ts is large at a high vehicle speed, the motor is damped. However, in the freely returning state of the steering wheel, the motor is not substantially damped since the steering torque Ts applied is substantially null.
The present invention has been achieved in an effort to effectively solve the above problem of the conventional motor-driven power steering system for automotive vehicles.